The usual method of insulating electrical apparatus, such as transformer coils and magnet actuation coils, is to encapsulate them with a resinous, liquid potting composition. This potting composition is generally an anhydride cured epoxy resin, which may contain up to about 2 parts of a silica filler per 1 part epoxy, as taught by Smith in U.S. Pat. No. 3,784,583. Use of over about 70 weight percent silica in the liquid composition presents problems of pourability, although use of large amounts of silica improves the electrical properties of the encapsulant.
Problems associated with a liquid composition, containing about 30 weight percent liquid resin, are relatively short shelf life and difficulty in bulk handleability. If a solvent is used with such large amounts of resin, then ecological problems may be presented in solvent removal during cure.
Large amounts of silica or sand in combination with a resin and curing agent have been used in various support systems. Fitko et. al., in U.S. Pat. No. 2,706,188, used up to 90 weight percent sand, 8 weight percent amine curing agent and 2 weight percent phenolic resin, to make a free flowing, dry, partially reacted resin coated sand composition, for use in shell molding processes for casting molten metals. Vondracek et. al., in U.S. Pat. No. 3,598,241, used up to 98 weight percent silica or sand and 2 weight percent amine catalyzed phenolic or epoxy resins, or peroxide catalyzed polyester resins, to make a free flowing, dry, resin coated sand composition, for use as a reverse osmosis membrane support tube.
What is needed in the electrical industry, is an easily handleable, pourable, dry potting composition which will have an excellent shelf life, and allow ease of insertion into complex geometries, and cure without major pollution problems.